The present invention relates generally to magnetic resonance imaging (MRI), and more particularly to a method and apparatus to reduce ghosting artifacts, resulting from orthogonal perturbation fields, in MR images acquired using fast imaging techniques.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field in the z direction, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is at or near the Larmor frequency, the net aligned moment, or xe2x80x9clongitudinal magnetizationxe2x80x9d, Mz, may be rotated, or xe2x80x9ctippedxe2x80x9d, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited spins after the excitation signal B1 is terminated and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (Gx Gy and G2) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting MR signals are digitized and processed to reconstruct the image using one of many well-known reconstruction techniques.
Imperfections in the linear magnetic field gradients (Gx, Gy and Gz) produce artifacts in the reconstructed images. It is a well-known problem, for example, that eddy currents produced by gradient pulses will distort the gradient fields and produce image artifacts. Methods for compensating for such eddy current errors are also well known and are disclosed in U.S. Pat. Nos. 4,698,591; 4,950,994; and 5,226,418, for example. It is also known that the gradients may not be perfectly uniform over the entire imaging volume, which may lead to image distortion. Methods for compensating this non-uniformity are described, for example, in U.S. Pat. No. 4,591,789.
Other than uncompensated eddy current errors and gradient non-uniformity errors that escape correction, it is often assumed that the magnetic field gradients (Gx, Gy, and Gz) produce linear magnetic fields exactly as programmed, thus spatially encoding the MR data accurately. With these gradients, the overall static magnetic field at location (x,y,z) is conventionally given as B0+Gx(x)+Gy(y)+Gz(z), and the direction of the field is usually thought to be along the z-axis. This description, however, is not exactly correct. As long as a linear magnetic field gradient is applied, the overall magnetic field direction is changed from the z-axis and its amplitude exhibits higher-order spatial dependencies (x2, y2, z2, z3, . . . ). These phenomena are a direct consequence of the Maxwell equations which require that the overall magnetic field satisfy the following two condition: {right arrow over (∇)}xc2x7{right arrow over (B)}=0 and {right arrow over (∇)}xc3x97{right arrow over (B)}≈{right arrow over (0)}. The higher-order magnetic fields, referred to as xe2x80x9cMaxwell termsxe2x80x9d (or Maxwell fields), represent a fundamental physics effect, and are not related to eddy currents or imperfection in hardware design and manufacture.
Many MR scanners still in use to produce medical images require several minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, improves image quality by reducing motion artifacts and enables dynamic and functional studies. There is a class of pulse sequences which can acquire an image in seconds, or even sub-second, rather than minutes.
One of these fast imaging techniques is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al. in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled xe2x80x9cRARE Imaging: A Fast Imaging Method for Clinical MR.xe2x80x9d A slight variation of the RARE sequence produces a fast spin echo (FSE) sequence which is used for clinical diagnosis in many commercial scanners. Images acquired using an FSE sequence are very susceptible to artifacts caused by eddy currents induced by the rapidly changing magnetic field gradients. While eddy current compensation techniques are adequate for scans performed with conventional MRI pulse sequences, it has been observed that image artifacts caused by eddy currents are frequently present in FSE scans.
Echo-planar imaging (EPI) is another ultrafast MR imaging technique which is extremely susceptible to system imperfections, such as eddy currents, B0 inhomogeneity, and gradient group delays. In the presence of eddy currents, for example, ghosting artifacts can considerably degrade the image quality and adversely affect EPI""s diagnostic value.
To minimize the ghosts created by such fast imaging techniques using echo trains, such as FSE and EPI, a common approach is to employ a reference scan prior to the actual image acquisition. In these reference scans, signals from a full echo train are acquired in the absence of the phase-encoding gradient. Each echo in the echo train is Fourier transformed along the readout direction to obtain a set of projections. Spatially constant and linear phase errors, xcfx860 and xcfx861, are then extracted from the projections, followed by phase corrections using xcfx860 and xcfx861, either during image acquisition, as in the case of FSE, or in image reconstruction, as in the case of EPI.
This type of phase correction assumes that spatially varying magnetic fields along the phase-encoding direction are negligible during the reference scans. However, when the Maxwell terms and other perturbation fields are considered, this assumption does not hold, especially when a strong gradient is used at relatively low static magnetic fields. In addition to the Maxwell terms, other factors that can cause perturbations to the reference scans, that can result in incomplete or erroneous phase corrections, include linear eddy currents from any gradient to the phase-encoding axis, a magnetic field inhomogeneity in the phase-encoding direction, and/or magnetic hysteresis that creates phase encoding direction field variations. Together with the Maxwell terms, these perturbations are herein referred to as orthogonal perturbation fields (OPFs).
In the presence of these fields, signal dephasing along the phase-encoding direction can introduce substantial errors in the constant and linear phase calculations that can lead to incomplete or erroneous phase correction. The perturbation of these terms to the reference scans may be evidenced by the fact that the aforementioned phase correction method works markedly well for axial EPI scans performed on a horizontal superconducting magnet, but not as well for sagittal and coronal scans. In the former case, the EPI readout gradient does not produce a quadratic Maxwell term on the phase-encoding axis, whereas in the latter cases, substantial Maxwell terms can be introduced.
It would therefore be desirable to have a technique to minimize the effects of OPFs on reference scans to thereby reduce ghosting.
The present invention relates to a method and system to reduce the effects of orthogonal perturbation fields (OPFs) in MR images by restricting the region of interest when acquiring a reference scan that overcomes the aforementioned problems.
The technique of the present invention involves limiting the region of interest when acquiring the MR reference scan to a relatively narrow band within the imaging subject. Preferably, the narrow band is selected parallel to the readout direction, or readout axis, centered about the MR magnet""s iso-center where the OPFs are minimal. Alternatively, the narrow band can also be restricted to a region of the field-of-view (FOV) where the OPFs are approximately constant in space. Two techniques are disclosed to limit the region of interest. In one of the techniques, the region of interest is limited by saturating nearby regions which results in two spatial saturation regions on either side of the desired region of interest to create a centralized, relatively narrow band in which reference scan data is acquired. Alternately, a single saturation band can be employed in instances where the object, or patient, is positioned asymmetrically along the phase encoding direction with respect to the iso-center of the MR magnet such that only a single saturation band results in a reduced, narrow band region of interest in the patient. In the other technique, instead of using a saturation pulse, a volume selection technique is employed to create the central band for the reference scan using a two-dimensional spatially selective RF pulse to select the narrow band, preferably, centered about the magnet""s iso-center or along the phase encoding direction at a point where the OPF variations are minimal with respect to the phase encoding direction. Using either technique, the width of the selected band in the region of interest must be wide enough to maintain a sufficient signal-to-noise ratio.
In accordance with one aspect of the invention, a method of reducing OPF effects in MR images includes acquiring raw MR data and transforming the raw MR data into MR image data, and acquiring an MR reference scan in the presence of the OPFs. The acquisition of the MR reference scan is limited to a region of interest having a relatively narrow band within the FOV. This technique includes extracting phase correction values from the MR reference scan and performing phase correction on the MR image data using the extracted phase correction values. An MR image can then be reconstructed using the corrected MR image data. By reducing errors from OPFs, the reconstructed MR image is displayed with reduced ghost artifacts.
In accordance with another aspect of the invention, a method of reducing ghost artifacts and minimizing phase errors in MR image acquisition includes acquiring MR data and acquiring an MR reference scan in the presence of OPFs in a limited region of interest parallel to the readout axis. The method includes determining at least one characteristic of the phase of the MR reference scan to determine phase correction values only within the region parallel to the readout axis and performing phase correction on the acquired MR data using the phase correction values.
In accordance with yet another aspect of the invention, an MRI apparatus is disclosed to create MR images with reduced ghost artifacts using a fast imaging technique. The apparatus includes an MRI system having a plurality of gradient coils positioned about a bore of a magnet to impress a polarizing magnetic field, an RF transceiver system, and an RF switch controlled by a pulse module to transmit RF signals to an RF coil assembly and thereby acquire MR images. The MRI apparatus includes a computer programmed to acquire raw MR data and transform the raw MR data to MR image data. The computer is also programmed to acquire an MR reference scan in the presence of OPFs and within a limited region of the FOV. The computer program then causes the computer to extract phase correction values from the MR reference scan and perform phase correction on the MR image data using the extracted phase correction values. An MR image is then reconstructed by the computer using the corrected MR image data. By reducing the OPF effects, the reconstructed image displays reduced ghost artifacts.
In accordance with yet another aspect of the invention, a computer program is disclosed for use with an MRI apparatus which, when executed by a computer, causes the computer and MRI apparatus to acquire MR data of a region of interest, reduce the region of interest in at least one dimension, and acquire an MR reference scan of the reduced region of interest in the presence of a frequency encoding gradient. The reduced region of interest provides a reduction of OPF effects on the MR reference scan in the at least one dimension. Phase correction values can then be obtained from phase characteristics of the MR reference scan, which in turn, are used to correct errors in the MR data. The system then reconstructs an MR image having reduced OPF effects and reduced ghost artifacts.
Various other features, objects and advantages of the present invention will be made apparent from the following detailed description and the drawings.